This disclosure relates to the method and apparatus for forming articles from thermoplastic materials, and more particularly to the method and apparatus for controlling the amount of crystallization by controlling the rate of spherulite growth in such materials during solidification cooling in a mold cavity.
Certain thermoplastic materials such as polyethylene therephthalate exhibit a strong tendency to crystallize within certain temperature ranges which are traversed as these materials cool from melt temperature within a mold cavity. Since the total amount of crystallization in the material is essentially the same regardless of the degree of constant pressure imposed upon the material, the extent of crystallinity has previously been controlled by temperature.
Typically, the amount of crystallization has been suppressed in a temperature-controlled method by rapidly quenching the material within the mold cavity and thereby rapidly traversing the crystallizable temperature range. This particular method, however, has its inherent drawbacks and limitations. Most notably temperature reduction in the material is dependent upon thermal conductivity of both the thermoplastic material and the mold. Larger articles obviously require greater time to cool and are thus subject to greater crystallization. More importantly, the temperature-controlled method does not provide the practical capability of significantly reducing crystallization to certain desirable levels for large articles.
In those situations where more extensive crystallization is desired, the temperature-controlled method may be successively used to achieve a desired level of crystallization. However, it will be appreciated that the thermoplastic material must remain in the mold for greater periods of time, resulting in an increased cycle time.
In short, the temperature-controlled methods for either suppressing or inducing crystal growth in the material present undesirable problems.